With the proliferation of small and medium sized personal computers located at a number of physically separated sites and used by a number of different users, there is also an accompanying need and desire to interface the individual computer sites into a large network in order to facilitate communications between and amongst each network site. As the number of sites in a network grows and the physical separation distance between networked sites grows, it is important to provide a network which will enable communication at a relatively high rate of speed in order to accommodate a large number of computers interconnected in one network.
One such large network is a token ring. In a token ring configuration, a number of local sites are serially connected by a transmission medium to form a closed loop. Information is then transmitted sequentially, as a stream of data called a message, from one active station to the next. Each station generally regenerates and repeats each message and serves as the means for attaching one or more devices to the ring for the purpose of communicating with other devices on the ring. A given site that has access to the transmission medium transmits information onto the ring, where the information then circulates from one station to the next. The information is accompanied by an address which designates the destination site to receive the message. The addressed destination site copies the information as it passes. The message travels around the serially interconnected stations and is finally returned to the initial source site which then removes the transmitted information from the ring.
A site gains the right to transmit its information onto the medium when it detects and captures token passing on the medium. The token is a control signal comprised of a unique symbol sequence that circulates on the medium in addition to transmitted data. After the detection of a token, the site detecting the token may remove the token from the ring. That site may then transmit one or more frames of information, and at the completion of its data transmission, issues a new token which provides other sites the opportunity to gain access to the ring. The length of time in which a site may occupy the medium before passing the token is controlled by a token-holding timer.
In addition to ring access being arbitrated by a token, multiple levels of priority are available for independent and dynamic assignment ring bandwidth, depending upon the relative class of service required. The classes of service are synchronous, asynchronous, or immediate service. For all classes of service, the allocation of a finite ring bandwidth occurs by mutual agreement among the users of the ring. The finite bandwidth is primarily divided into the formentioned classes as defined above into synchronous and asynchronous portions. Asynchronous communication defines a class of data transmission service in which all requests for service contend for a pool of dynamically allocated ring bandwidth and response time. Synchronous communication defines a class of data transmission service in which each requester is preallocated a maximum bandwidth and guaranteed a response time not to exceed a specific delay. Immediate data transmission is generally used only for extraordinary applications such as ring recovery.
When a number of network sites are distributed over distances of several miles, the transmission medium over which that data is transmitted becomes of much greater importance. One such transmission medium over which a number of network stations may be interconnected is a fiber optic medium. In a fiber optic medium, optical signals from light-generating transmitters are propagated through optical fiber wave-guides to light-detecting receivers. The above named classes of data transmission are implemented in a standardized fiber optic network defined as a fiber distributed data interface (FDDI). The FDDI generally consists of a number of layers: a physical layer, which defines the physical requirements of the data interface; a data layer, which defines fair and deterministic access to the medium as well as a common protocol to ensure data integrity; and a station management layer, which defines the control necessary at the station level to manage the processes underway in the various FDDI layers. This invention is directed at an improvement in the data link layer. More specifically, this improvement is directed toward the media access control (MAC) which provides access to the medium and verification of transmission sequences. The information as described above including a definition of the MAC may be found in a document entitled Fiber Distributed Data Interface (FDDI)--Token Ring Media Access Control (MAC), American National Standards Institute (ANSI) document number ANSI X3.139-1987, and is herein incorporated by reference.
Using synchronous channels, an FDDI network provides both bounded transmission delays and a guaranteed bandwidth for the synchronous traffic. The FDDI standard is designed for a 100 Mbps token ring network implemented on a fiber optic medium. Because of its high speed, FDDI alleviates the bandwidth saturation problem of current slower ethernet and token ring systems. Furthermore, the synchronous traffic support capability of FDDI also makes it ideal for supporting multi-media applications which integrate the transmissions of ordinary data and digital voice and video signals. Using synchronous channels, a FDDI network provides both bounded transmission delays and a guaranteed bandwidth for synchronous traffic. The transmission delay is controlled by a target token rotation timer (TTRT) which guarantees each node will have an opportunity to transmit synchronous messages at least once within a predetermined time period. It also ensures that the average time between consecutive opportunities to transmit does not exceed the TTRT value. Once a node has access to the network, the bandwidth of synchronous traffic that that particular node may transmit is guaranteed by assigning to that node a portion of the TTRT value for the entire token ring, called the high priority token holding time for node i, Hi. This yields a guaranteed bandwidth for a node in which to transmit synchronous messages. However, a node usually can not fully utilize the guaranteed bandwidth due to the transmission delay requirement of synchronous messages. This invention is directed at a method of improving the allowable bandwidth in which to transmit synchronous messages.